Many chemical reactors are made as shell-and-tube heat exchangers, with many parallel, vertical tubes extending from a top tube sheet to a bottom tube sheet. During use, the tubes typically contain catalyst. Periodically, the reactor is shut down, the catalyst is removed, the tubes are cleaned out, and generally new catalyst is loaded.
In order to clean out the tubes of these heat exchangers, grit blasting often is used. Grit blasting uses an abrasive material, which may be sand, other mineral-type abrasives, dry ice pellets, or other similar abrasive material, to clean the tubes. In that procedure, a grit blasting nozzle is generally inserted into the tube by hand. It is important for the operator to keep the grit blasting nozzle axially aligned with the reactor tube so it directs the abrasive material down the tube. If the nozzle gets cocked off of the longitudinal axis of the tube, directing the blast of abrasive material against the wall of the tube, or if the grit blasting time period is too long, it is possible to blast a hole through the wall of the tube, which is not desirable.
The gas used for grit blasting of sand or other mineral-type abrasives is commonly 100% oil-free dessicant dried air with a dew point of approximately minus 40 degrees Fahrenheit, while the gas to propel the frozen carbon dioxide abrasive material (dry ice pellets) may have a dew point that is even lower.
Grit blasting may be hazardous to personnel performing the cleaning, and serious injury including death can result if the flow of the abrasive material is not properly controlled. For example, if the grit blasting nozzle gets loose outside of the tube while forcefully blowing abrasive material, it can do other damage to the reactor and attached and surrounding peripherals or to the people working in the reactor.
Grit blasting is often followed by the insertion of a swab. Swabs, sometimes referred to in the trade as “pigs”, can be made of foam, felt, cloth, or similar materials; there may be abrasive materials deposited or embedded in the material, and there may be various combinations of materials, with or without abrasives added. The swabs are inserted into the tubes and are pushed through mechanically, or are blown through with gas pressure. A swab wipes the sidewall of the tube as it passes through the tube, thereby cleaning off the wall of the tube. It takes time to load all the swabs and pass them through their respective tubes. Since there may be thousands of tubes in a reactor, it would be helpful to be able to speed up the process and to have a way of keeping track of which tubes have been swabbed. Also, a swab may be left in a tube accidentally, which adversely affects the functioning of the reactor.